It is known to apply phosphate coatings to metal surfaces by immersion or flow coating with aqueous acidic zinc phosphate solutions which contain chlorate or chlorate and nitrate as accelerators. In West German Offenlegungsschrift No. 21 06 626, chlorate-containing zinc phosphate solutions are disclosed in which the weight ratio of P.sub.2 O.sub.5 :Zn is maintained within the range of about 1:0.2-0.7. Similarly, from West German Patent Specification 10 96 152, it is known to use calcium irons in such solutions to improve the phosphate layer formation. In either case, however, undesirably long treatment times are required to obtain the formation of the desired continuous phosphate layer on the metal surfaces. Moreover, the phosphate layers obtained from the use of such processes have not always been satisfactory, particularly when used in subsequent cold forming operations, due to the undesirably low weight or thickness of the phosphate layer.
In an attempt to overcome these difficulties, phosphate coating solutions have been proposed in West German Offenlegungsschrift No. 25 40 685 which are acidic zinc phosphate solutions containing chlorate and nitrate accelerators and in which the weight ratio of P.sub.2 O.sub.5 :Zn is 1:0.8-4.0. With phosphating baths of this type, a phosphate layer is obtained having a crystalline structure which is somewhat softer than normal. This results in an increase in the absorptive capacity of the layer for a subsequently applied lubricant, such as is obtained by treatment with an aqueous soap solution, prior to a cold forming operation. Particularly good results have been obtained with a phosphatizing bath of this type which contained at least 6 g/l of zinc, at least 5 g/l P.sub.2 O.sub.5, at least 1 g/l ClO.sub.3, at least 8 g/l NO.sub.3, with a total acid number of from 20 to 80 and in which the weight ratio of P.sub.2 O.sub.5 :Zn:NO.sub.3 :ClO.sub.3 was equal to 1:1.5-4.0:2.0-6.0:0.03-2.0 and the weight ratio of free P.sub.2 O.sub.5 :total P.sub.2 O.sub.5 was equal to 0.2-0.6:1. In operation, the excellent coating results with this bath were obtained by replenishing the bath with a phosphating solution in which the weight ratio of P.sub.2 O.sub.5 :Zn:NO.sub.3 :ClO.sub.3 was 1:0.4-0.8:0.1-0.6:0.15-0.6 and the weight ratio of free P.sub.2 O.sub.5 :total P.sub.2 O.sub.5 was 0.2-0.7:1.
In spite of the fact that the phosphate coatings obtained from the operations of such phosphatizing processes have very good properties and provide a considerable improvement over prior processes, they are sometimes subjected to cold forming operations which require even greater thickness and coalescence of the phosphate layer to the metal substrates than can be obtained from such processes.
It is, therefore, an object of the present invention to provide an improved phosphating composition and process which will form phosphate coatings that are capable of meeting the most stringent requirements presently known in regard to the thickness of the phosphate layer and its coalescence to the metal substrate.
This and other objects will become apparent to those skilled in the art from the description of the invention which follows.